1. Field of the Invention
This invention relates to optically anisotropic pitches which are generally suited for manufacturing high performance carbon fibers (the term "carbon fibers" being used in this specification to refer, unless particularly specified, not only to carbon fibers but also to graphite fibers) and, more particularly, to optically anisotropic pitches suited for manufacturing high compressive strength carbon fibers and a method of manufacturing high compressive strength carbon fibers using such optically anisotropic pitches.
The high compressive strength carbon fibers obtainable according to the present invention can be suitably used as reinforcement fibers for composite materials used in various industrial fields such as space and aircraft, automotive, construction and so forth industries.
2. Prior Art
In space and aircraft, automotive, construction and various other industrial fields, PAN- or rayon-based carbon fibers, which have high tensile strength and high tensile elastic modulus, for instance, have heretofore been used extensively as reinforcement fibers for light, high strength and high elasticity composite materials. These carbon fibers, however, require expensive raw materials. In addition, their carbonization yield is inferior. Therefore, they are posing various economical problems.
On the other hand, pitch-based carbon fibers, which are obtainable from petroleum pitch or coal pitch, require inexpensive raw material cost and also have high carbonization yield. For these reasons, extensive researches and investigations of this type of carbon fibers have recently been conducted. It has been said, however, that the pitch-based carbon fibers, although superior in the tensile elastic modulus to the PAN- and rayon- based carbon fibers, are inferior in the compressive strength and therefore can find only limited applications.
For the manufacture of carbon fibers, there have been proposed many optically anisotropic pitches obtainable from, for instance, petroleum pitch or coal pitch, and also many methods of producing such optically anisotropic pitches. However, there are very few proposals with the aim of improving the compressive strength of pitch-based carbon fibers. Particularly, there is substantially no proposal with the aim of the compressive strength improvement with optically anisotropic pitch.
Only Japanese Patent Application Laid-Open No. 14023/1990 shows carbon fibers excellent in the compressive strength and obtainable from a pitch with an optically anisotropic phase of 40% or below and also a method of manufacturing such carbon fibers. In this case, however, the preparation of the pitch requires a large number of steps such as hydrogenation of the material pitch, thermal treatment of the hydrogenated pitch and two-stage solvent extraction of thermally treated pitch. Therefore, it is thought that high cost of manufacture is necessary. In addition, the laid-open does not show any method of measuring the compressive strength of obtained carbon fibers, and therefore the obtainable effects are uncertain.
Japanese Patent Publication No. 4558/1979 shows optically anisotropic pitches which are obtained through sole thermal treatment. In this case, the characteristics of the optically anisotropic pitches are specified. However, although the number-average molecular weight is shown to be about 800 to 900, the molecular weight distribution is thought to be considerably broad, because of the manufacture through the sole thermal treatment. Therefore, with the disclosed optically anisotropic pitches it is impossible to expect improvement of the compressive strength of carbon fibers.
Japanese Patent Publication No. 57715/1989 shows specification of the molecular weight distribution of optically anisotropic pitch. However, since the molecular weight distribution can not be sufficiently controlled, there is no guarantee for improvement of the carbon fiber compressive strength. Further, concerning examples disclosed in the publication, the yield of optically anisotropic pitch is low, and the softening points of obtained optically anisotropic pitches are high.
As shown, with the optically anisotropic pitches obtainable by the prior art thermal treatment process, limitations are imposed on the ranges of control of the composition, number-average molecular weight, molecular weight distribution and so forth. Therefore, it is impossible to obtain an optically anisotropic pitch, which is suited for manufacturing high compressive strength carbon fibers as according to the present invention.
Aside from the thermal treatment process, the solvent extraction process may permit control of the composition, number-average molecular weight, molecular weight distribution, etc. of the pitch. However, in order to more accurately control and specify these characteristics, it is necessary to specify the material.
In the prior art solvent extraction processes, no material is specified. Therefore, the obtainable optically anisotropic pitches have different characteristics from those of the optically anisotropic pitch, which is suited for manufacturing the high compressive strength carbon fibers as according to the present invention.